Support parsing and caching AppStream categories in solvable metadata

TL;DR:
AppStream categories installed by libzypp-plugin-appdata aren’t stored in the libsolv cache, so frontends like zypper and Myrlyn cannot query them. Requesting libsolv support to parse and store these categories as solvable keywords or attributes.

Hello libsolv maintainers

I’m following up on a discussion in libzypp issue #720
about exposing AppStream categories via the libzypp API.

It was suggested that libsolv would need to be able to parse this metadata into the solvable cache (for example, as package keywords). If the categories are cached in this way, frontends could then offer searches for them.

I don’t have a full understanding of the internal details, but I’m hoping support could be added in libsolv to parse and store AppStream categories (or similar metadata) as solvable keywords or attributes. Once cached, libzypp and frontends like zypper and Myrlyn could provide meaningful, category-based searches, improving package discoverability and software center integration.

This came about as the result of a discussion at
https://forums.opensuse.org/t/myrlyn-rpm-groups-view-is-back-do-we-want-it/187677

I hope this makes sense...I'm happy to provide further details or examples if that would help.

Thank you for considering this enhancement.

The issue names no libsolv files, entry points, or tests. Start by reviewing libzypp issue #720 and the AppStream metadata produced by libzypp-plugin-appdata, then trace how repository metadata is parsed and cached. Done means AppStream categories are stored in solvable metadata in a form that libzypp and frontends can query.